H. Kakemoto

Ultrawide range dielectric spectroscopy of BaTiO3-based perovskite dielectrics

Ultrawide range dielectric spectra from the kilohertz to terahertz range of BaTiO3 (BT), Ba(Zr0.25Ti0.75)O3 (BZT), (Ba0.6Sr0.4)TiO3 (BST), and SrTiO3 ceramics were presented by analyzing dielectric permittivity and IR reflectivity data. It was found that the permittivity of the ST was determined only by the ionic polarization while that of the BT was determined by the ionic polarization as well as the dipole polarization due to the domain contribution. The high permittivity of the BZT ceramics was attributed to the dipole polarization of polar nanoregions in the relaxors. The dipole and ionic polarizations overlapped in the BST.

Preparation of nm-sized BaO3 particles using a new 2-step thermal decomposition of barium titanyl oxalate

To obtain inpurity-free and nm-sized barium titanate (BaTiO3) particles, a new 2-step thermal decomposition method from barium titanyl oxalates (BaTiO(C2O4)2 · 4H2O) was proposed. At the 1st step, BaTiO(C2O4)2 · 4H2O was annealed at 400°C for 1 h in the O2 flow. The annealing temperature of 400°C was chosen for the following reasons; (1) no formation of BaCO3 and TiO2 and (2) the complete removal of H2O and other carbon species. This compound obtained at 400°C was amorphous phase, and its chemical composition was BaCO3-TiO2. When this compound was annealed in air at higher temperatures, the large BaTiO3 particles were prepared with by-products such as BaCO3. Thus, at the 2nd step, to prevent the crystal growth and the formation of BaCO3, this compound was annealed above 600°C in vacuum. Finally, the BaTiO3 single crystals with a size with 16.5 nm were prepared around 620°C. These BaTiO3 fine particles were characterized using various methods to investigate defects and impurities in the particles. As a result, it was confirmed that there was no impurity in the BaTiO3 lattices.

Domain wall engineering in barium titanate single crystals for enhanced piezoelectric properties

For the [111] poled barium titanate (BaTiO 3 ) single crystals with the engineered domain configuration, it was clearly observed that the piezoelectric properties increased with decreasing domain sizes. To explain the phenomenon, the multidomain single crystals were regarded as the composite of (a) a distorted domain wall region and (b) a normal tetragonal domain region. Using a 2-phases model, the piezoelectric properties from the domain wall were estimated. As a result, ultrahigh piezoelectric constants over 8,000 pC/N were expected from the domain wall region. Moreover, this study suggested that it is possible to obtain the lead-free piezoelectric materials with the d 31 and d 33 over 1,000 pC/N, when the domain sizes can decrease below 1 μ m.

Domain Wall Engineering in Lead-Free Piezoelectric Grain-Oriented Ceramics

[110]-oriented barium titanate (BaTiO3) ceramics were prepared by templated grain growth (TGG) method. As a result, BaTiO3-grain-oriented ceramics with a high density of more than 96% were successfully prepared despite various degrees of orientation along the [110] direction, F 110 values, from 0 to 98%. The d 31 values were almost constant at −50 pC/N despite various F 110 values, while the d 33 values increased with increasing F 110 values, and at around an F 110 of 85%, d 33 reached a maximum of 788 pC/N.

Enhanced Piezoelectric Property of Barium Titanate Single Crystals by Domain Wall Engineering Using Patterning Electrode

For the [111] oriented barium titanate (BaTiO3) single crystals, the patterning electrode was used to induce the finer engineered domain configurations with domain size below 5 μ m. The poling treatment was performed at 134.0°C under electric fields below 6 kV/cm to inhibit the burning of the patterning electrode with photoresist. As the results, the gradient domain sizes from 3 μm (high voltage side) to 8–9 μ m (ground side) were induced into the 31 resonator along thickness direction. For this resonator, the d31 was measured at −243.2 pC/N using a resonance-antiresonance method.

Crystal Growth of Lithium-Doped Silver Niobate Single Crystals and Their Piezoelectric Properties

Silver lithium niobate (Ag 0.9 Li 0.1 NbO 3 , ALN10) single crystals were grown by a slow cooling method without flux under oxygen flow. By optimizing growth conditions, the ALN10 crystals with sizes of ca. 30 mm cube were successfully grown. Its crystal symmetry was investigated by high energy X-ray diffraction, and the space group at 24°C was assigned to the ferroelectric Pc2 1 b orthorhombic symmetry. The ALN10 crystals were oriented along the [010]o and the [110]o directions, and their piezoelectric properties were measured using the 31 resonators.

Composite Structure and Size Effect of Barium Titanate Nanoparticles

Almost impurity and defect-free barium titanate (BaTiO3) nanoparticles with various sizes from 20 to 430 nm were prepared using 2-step thermal decomposition method. The nano-structures of these particles were analyzed using a synchrotron radiation X-ray diffraction (XRD). As a result, it was found that the BaTiO3 nanoparticles had composite structure consisted of (a) internal tetragonal layer, (b) Gradient-Lattice-Strain Layer (GLSL) and (c) surface cubic layer.

Temperature Dependence of Dielectric Properties of BaTiO3/SrTiO3 Artificial Superlattices

BaTiO3/SrTiO3 artificial superlattices were fabricated by the molecular beam epitaxy process. The superlattice with the 10-periodic structure was mostly distorted in the film thickness direction, and showed the highest dielectric permittivity than other periodic superlattice. The temperature dependence of dielectric properties was measured in the range from room temperature to +150 degrees at 1 GHz. Compared with Ba x Sr1−x TiO3 solid solution ceramics or BaTiO3 ceramics, there was almost no temperature dependence of dielectric permittivity. As a result, it was cleared that induced lattice distortion played an important role to determine the dielectric properties of artificial superlattices.

Fabrication and piezoelectric properties of (K 0.5 Na 0.5 )NbO 3 -based ceramics doped with Bi-perovskites

The effect of the addition of Bi-perovskites to (Li_0.04K_0.52Na_0.44)(Nb_0.84Ta_0.1Sb_0.06)O₃ (LF4) ceramics was studied to improve the properties of lead-free piezoelectrics. Ceramics with the relative density above 96% were successfully obtained by a solid state reaction process. The changes in lattice parameters indicated that the vacancy concentration at the A-site of the perovskite structure and the tetragonality of the crystal lattice were reduced with the addition of BiFeO₃. The addition of BiFeO₃ changed the LF4 to a soft-piezoelectric. The electrical insulation of the LF4 was improved with the addition of BiFeO₃, which made a full poling possible under high electric fields. A few mol% of BiFeO₃ was effective to improve the remanent polarization of the LF4 ceramics and a piezoelectric d₃₃ constant of about 358 pm/V was obtained in the 0.4 mol% doped ceramics.

Preparation of Potassium Niobate Crystals with Fine Engineered Domain Configurations and Their Enhanced Piezoelectric Properties

Potassium niobate (KNbO 3 ) single crystals were grown by a top-seeded solution growth (TSSG) method. At first, the electric field was applied along [001]c (cubic notification system) direction of KNbO 3 crystals to induce the engineered domain configurations into KNbO 3 crystals. Prior to domain engineering, the piezoelectric properties of [001]c oriented KNbO 3 single-domain crystals were measured. These measurement values were completely consisted with the calculated d 31 and d 32 . Finally, the fine engineered domain configurations were induced into KNbO 3 crystals. As a result, piezoelectric properties increased with decreasing domain sizes of the engineered domain configuration. However, the symmetry of the KNbO 3 crystals was, and there were four kinds of domain structures such as 90°, 180°, 60° and 120° domains. Thus, the engineered domain structure induced in this study was very complicated structure, and the piezoelectric properties were also depended on domain pattern and kinds of domain walls.